A cyclobutane derivative compound (IV), which is expected to be useful as an antiviral drug, may be synthesized by, for example, the following method (refer to EP-A-358154). ##STR4## wherein R.sup.10 represents a hydrogen atom or a protecting group; R.sup.11 represents an alkyl group having 1 to 5 carbon atoms, an aralkyl group or a cyclic alkylene group having 2 or 3 carbon atoms which is formed by binding two R.sup.11 groups to each other;
R.sup.12 represents a hydrogen atom, an alkyl group having 1 to 5 carbon atoms, a protected hydroxyalkyl group having 1 to 5 carbon atoms or a protected carboxyl group;
R.sup.13 represents a hydrogen atom, an alkyl group having 1 to 5 carbon atoms, an alkoxy group having 1 to 5 carbon atoms or an aralkyloxy group;
A represents a straight-chain or a branched alkylene group having 2 to 5 carbon atoms;
Y represents an oxygen or a sulfur atom; and
Z represents an optionally substituted methylene group or an oxygen or a sulfur atom.
However the aforesaid reaction comprising reacting a compound represented by the general formula (II) with a compound represented by the general formula (V) to thereby give a compound represented by the general formula (VI) cannot smoothly proceed unless the R.sup.13 group in the compound of the general formula (V) is a hydrogen atom. Further, the compound of the general formula (VI) thus obtained should be carefully handled since it is relatively unstable to acids.
Cyclobutane derivatives and condensed ring compounds having a cyclobutane ring are useful as drugs, agricultural chemicals, perfumes as well as starting materials and intermediates for the preparation thereof. However it is difficult to produce these cyclobutane derivatives and condensed ring compounds having a cyclobutane ring. In particular, no practical process has been known hitherto for preparing these compounds which are optically active.